1. Field of the Invention
The present invention relates to a method for producing high purity silica powder which can be fused to form transparent, bubble-free particles. More particularly, the invention relates to a method of recovering such fusible high purity silica powder from an unsaturated solution of ammonium fluosilicate.
2. Description of Related Art
Silica powder typically is produced in a number of ways to serve a variety of purposes. For example, silica can be produced by hydrolysis of a silicon tetrahalide or other silicon-containing compounds, such as tetraethylorthosilicate. These methods typically are not completely satisfactory. Hydrolysis of silicon tetrachloride, for example, can require days to form a gel, while tetraethylorthosilicate is a very expensive raw material.
Silica products produced by these methods typically are suitable only for selected uses. As an example, silica made by ammoniation of ammonium fluosilicate, for example, as taught in U.S. Pat. No. 1,903,187, is useful as adsorbent, but cannot readily be fused to form transparent, bubble-free particles suitable for use in encapsulation of electronic parts or other high-purity, high-density uses described below. According to this patent, ammonium fluosilicate produced during recovery of metals from silicates is ammoniated to produce silica acid precipitate.
U.S. Pat. No. 2,768,063 also discloses a method for producing silica by ammoniating ammonium fluosilicate solution. Ammonium fluosilicate solution which has been filtered to remove insoluble matter is introduced into an excess of aqueous ammonium solution and finely divided amorphous silica is precipitated. The silica is washed and utilized as the "frosting" for the inside of light bulbs. The patent indicates that the temperature of both solutions should be between about 25.degree. and 37.degree. C., and it is important to add the ammonium fluosilicate solution to the ammonium solution. Adding the ammonium solution into the ammonium fluosilicate solution is said to produce too much gel.
U.S. Pat. No. 3,271,107 discloses a process for producing a silica used in pigments by reacting fluosilicic acid with ammonium hydroxide in two stages. Fluosilicic acid is a by product of, e.g., phosphoric acid manufacture, and is produced when silicon tetrafluoride liberated during concentration of phosphoric acid is absorbed in water. Other fluosilicic acid sources are known to those skilled in the art. In the first stage, a less-than-stoichiometric quantity of ammonium hydroxide is added to fluosilicic acid with high agitation to produce a slurry having a pH of between 6.0 and 8.0 containing minute silica particles. The unreacted fluosilicic acid in this slurry then is reacted with sufficient ammonium hydroxide to provide a final pH between about 8.3 and 9.0. Pigment quality silica precipitate then is separated from the slurry.
U.S. Pat. No. 3,021,194 discloses a process for producing ammonium bifluoride from fluosilicic acid and ammonium fluoride without undue loss of ammonia or fluorine. Concentrated fluosilicic acid is reacted with ammonium fluoride, or a mixture of ammonium fluoride and sodium or potassium fluoride, to produce aqueous ammonium acid fluoride (ammonium bifluoride) solution and solid alkali fluosilicate, including ammonium fluosilicate. After separating the solution from the solid alkali fluosilicates, solid ammonium bifluoride is recovered by evaporatively concentrating the solution. Alkali metal fluosilicates can be recovered and sold, or can be converted to alkali fluorides by reaction with additional ammonia. Ammonium fluoride is produced and hydrated silica is precipitated by this ammoniation. The silica is indicated for use as a filler, a flatting agent, or as an insecticide provided it contains some sodium fluoride.
Certain uses of silica require very high purity material. For example, silica used in the encapsulation or packaging of electronic computer chips must have extremely low levels of metal impurities. Typical of these uses is very large scale integrated (VLSI) microchip applications, where chip manufacturers require silica having extremely low concentrations of certain radioactive elements. For example, uranium and thorium concentrations must be on the order of less than 1 part per billion (ppb). The maximum acceptable level of ionic impurities, including cations such as boron, calcium, cobalt, chromium, copper, iron, potassium, magnesium, manganese, sodium, nickel, vanadium, and zinc, and anions containing phosphorus and sulphur, is less than 10 parts per million (ppm), and often is below 1 part per million. The concentration of halogens also should be minimized to reduce chip corrosion and increase chip life.
Other uses for high purity silica material include precision laser optics, fiber optics, and advanced ceramics. These requirements now are satisfied predominantly by natural silica sources such as quartz. Although natural quartz is a crystalline form of silica, such quartz can be made amorphous by fusion techniques known to those skilled in the art. Thus, modified quartz, often called "fused quartz," suitably is used when amorphous silica is required. Unfortunately, prior art processes for recovering silica from contaminated fluosilicic acid starting materials, such as by-product fluosilicic acid recovered from phosphate rock acidulation, have not been satisfactory for producing a product satisfying these stringent purity requirements.
U.S. Pat. No. 4,465,657, for example, discloses a process for producing a purified silica from impure fluosilicic acid which basically uses the procedure of the earlier U.S. Pat. No. 3,271,107. Fluosilicic acid is reacted in a first step with a less--than--stoichiometric quantity of ammonium hydroxide to convert some of the acid to ammonium fluoride and silica. The silica precipitate thus produced removes metal ion impurities, presumably at least in part by adsorption, from the residual fluosilicic acid solution. The silica precipitate is separated, and the remaining solution having a lower level of impurities then is reacted in a second stage with additional ammonium hydroxide to produce a purified silica precipitate. Optionally, the residual fluosilicic acid solution from the first precipitation stage may be treated with an ion exchange or chelating agent to purify the solution further prior to formation of the silica precipitate in the second precipitation stage.
A particular drawback of this procedure is that from 40 to 75 percent of the available silica in the fluosilicic acid is used as the vehicle for removing impurities. Thus, only 25 to 60 percent of the silica values of the fluosilicic acid actually can be recovered in a purified form. Moreover, there is a tacit admission that the two step process does not produce a satisfactory product since it is preferred to treat the solution from the first step process with an ion exchange or chelating agent prior to the second precipitation step.
European Patent Application 0,113,137 attempts to avoid the loss in yield of U.S. Pat. No. 4,456,657 by adding a chelating agent directly to the impure fluosilicate acid solution. Purportedly, the chelating agent improves the purity of the first silica precipitate by sequestering or chelating multivalent metal ions in the solution before ammoniation. Ion exchange also has been used for the same purpose. However, these techniques tend to introduce other impurities, such as alkali metal ions, into the precipitated silica. Additionally, these prior art purification processes rely upon cationic exchangers and metal chelating agents and thus cannot satisfactorily remove the phosphorus and sulphur impurities generally present as anionic species (SO.sub.4.sup.-2 and PO.sub.4.sup.-3) in the fluosilicic acid by-product solutions typically recovered from the acidulation of phosphate rock. Nor can anionic exchange agents be used because the anionic exchange agents significantly decrease the recovery of silica.
Silica produced in accordance with these methods is not satisfactory for use in producing high purity, transparent, bubble-free particles because the silica product contains too many impurities. With respect to silica produced by ammoniating ammonium fluosilicate, the subsequently fused particles are not transparent and bubble-free. Methods known in the art for producing fusible silica are complex and difficult to carry out. One alternative, natural quartz, is very expensive and reserves are limited. Further, natural quartz typically is not acceptable for high purity fused product unless it is purified.
Japanese Patent 85(60)/42218 teaches a method of producing high purity silica suitable for electronic uses, for use as a filter for plastic resin, for use in adhesives, and the like. An aqueous solution of an alkali silicate is ultrafiltered to remove colloidal-sized particles. The filtered solution then is purified first with an acidic cation exchange resin, and then with an OH-type anion exchange resin, to obtain a purified silica sol. The purified silica sol is contacted with a precipitant such as ammonium chloride, ammonium nitrate, or ammonium carbonate to cause silica to precipitate. Precipitated silica is collected, then heated.
Soviet Union Patent 776,994 discloses a method for producing transparent, bubble-free quartz glass. Carbon dioxide is bubbled through a sodium metasilicate solution. A precipitate is formed which is washed, and then treated in acid at 130.degree.-200.degree. C. for at least about four hours. Treated precipitate subsequently is filtered, washed, briquetted, fired, and ground. The ground particles are vibro-sorted to segregate a fraction having a density between 1.4 and 1.6 g/cm.sup.3, and then are fused to obtain quartz glass.